1. Technical Field of the Invention
The present invention generally relates to communication networks. More particularly, and not by way of any limitation, the present invention is directed to a protection scheme for such networks.
2. Description of Related Art
A network operator typically takes into consideration multiple objectives when routing traffic through a network. One objective may be to minimize cost. Another objective may be to minimize transmission impairments. A third objective may be to maximize the possibility that the network can be restored in the event of a failure thereof.
Generally, there are three types of restoration schemes: dedicated restoration, shared restoration, and best effort restoration. In dedicated restoration, the capacity of a protection, or restoration, path is reserved for an individual demand. In shared restoration, the restoration capacity is reserved, but shared across multiple demands. In best effort restoration, no reservation is made and restoration capacity is allocated in real time on an as-available basis. The first two classes of restoration both have guaranteed restoration in the event of a single failure; however, they differ in restoration time, as shared restoration requires real-time path setup.
A system has been developed that can work as part of an optical cross-connect to perform distributed mesh restoration. This system uses GMPLS protocols including OSPF, LMP, and RSVP, to restore traffic in the event of a network failure. An intelligent mesh restoration scheme is based on diversely routed service and restoration paths.
An intelligent mesh routing algorithm computes disjoint paths for an end-to-end connection demand. The path computation is based on availability of capacity in such a way that overall network resource utilization is optimized. The network resource optimization not only enables the network to increase the amount of traffic carried, it enables the more even distribution of traffic across the network so that there are no bottlenecks. Since the path computation is distributed, the route utilization optimization is performed by dynamically tuning link weights.
In mesh restoration, protection paths may be predefined; however, the cross-connections along the paths are not created until a failure occurs. During normal operation, the optical channels reserved for protection are not used. When the capacity is only “soft reserved”, the same optical line can be shared to protect multiple lightpaths. Upon an actual link failure, the ingress and egress nodes of each path interrupted by the failure transmit a request to the nodes along the respective protection path to establish the cross-connections for the disconnected path. Once the cross-connections are established, the ingress and egress nodes restore the connection to the new path.
The concepts of link failure and path restoration are illustrated generally with reference to FIGS. 1A-1D. Referring first to FIG. 1A, illustrated therein is a generalized multi-protocol label switched (“GMPLS”) optical transport network 100 comprising a plurality of nodes 102A-102F interconnected by links 104A-104J. A working path through the network 100 is represented by a heavy black line designated by a reference numeral 106 and traverses the network via the nodes 102B, 102A, 102C, and 102E, and the links 104A, 104G, and 104F. Assume now that the link 104A fails for some reason and the traffic on the path 106 must be rerouted. According to mesh restoration techniques, there are several alternative paths, which are illustrated in FIGS. 1B-1D.
In particular, FIG. 1B illustrates a first alternative path, which is represented by a heavy black line designated by a reference numeral 110 and which traverses the network 100 via the nodes 102B, 102D, 102C, and 102E, and the links 104C, 104D, and 104F.
FIG. 1C illustrates a second alternative path, which is represented by a heavy black line designated by a reference numeral 120 and which traverses the network 100 via the nodes 102B, 102C, and 102E, and the links 104B and 104F.
FIG. 1D illustrates a third alternative path, which is represented by a heavy black line designated by a reference numeral 130 and which traverses the network 100 via the nodes 102B, 102C, 102F, and 102E, and the links 104B, 104J, and 104I.
In each of the restoration path examples illustrated in FIGS. 1B-1D, it will be recognized that sufficient capacity must have been previously reserved on the links used to construct the alternative, or protective, paths in order for restoration to be accomplished successfully.
Because, as previously noted, the optical channels reserved for protection are not used to carry traffic, it is advisable to reserve as few channels as possible, while making sure that enough channels are reserved to provide real protection in the event of a link failure. The objective, therefore, is to determine the optimum number of protective channels to reserve for each link.